A recently developed type of energy conversion device comprises: (A) an anodic reaction zone (i) which contains a molten alkali metal anode-reactant in electrical contact with an external circuit, and (ii) which is disposed interiorly of a tubular cation-permeable barrier to mass liquid transfer; (B) a cathodic reaction zone (i) which is disposed exteriorly of said tubular cation-permeable barrier, and (ii) which contains an electrode which is in electrical contact with both said tubular cation-permeable barrier and said external circuit; (C) a reservoir for said molten alkali metal which is adapted to supply said anode-reactant to said anodic reaction zone; and (D) a tubular ceramic header (i) which connects said reservoir with said anodic reaction zone so as to allow molten alkali metal to flow from said reservoir to said anodic reaction zone, (ii) which is sealed to said tubular cation-permeable barrier, and (iii) which is impervious and nonconductive so as to preclude both ionic and electronic current leakage between the alkali metal reservoir and the cathodic reaction zone. Among the energy conversion devices falling within this general class are: (1) primary batteries employing electrochemically reactive oxidants and reductants in contact with and on opposite sides of the tubular cation-permeable barrier; (2) secondary batteries employing molten electrochemically reversibly reative oxidants and reductants in contact with and on opposite sides of the tubular cation-permeable barrier; (3) thermoelectric generators wherein a temperature and pressure differential is maintained between anodic and cathodic reaction zones and/or between anode and cathode and the molten alkali metal is converted to ionic form passed through the cation-permeable barrier and reconverted to elemental form; and (4) thermally regenerated fuel cells.
A particularly preferred type of secondary battery or cell falling within the type of energy conversion device discussed above is the alkali metal/sulfur or polysulfide battery. During the discharge cycle of such a device, molten alkali metal atoms, e.g., sodium, surrender an electron to the external circuit and the resulting cation passes through the tubular barrier and into the liquid electrolyte in the cathode reaction zone to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surface of the electrode by reaction of the cathodic reactant with electrons conducted through the electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the electrode in the vicinity of the cation-permeable barrier. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the tubular barrier and the alkali metal cations can pass out of the tubular barrier into the liquid electrolyte and combine to form alkali metal polysulfide near the barrier. As the device begins to discharge, the mole fraction of elemental sulfur drops while the open circuit voltage remains constant. During this portion of the discharge cycle as the mole fraction of sulfur drops from 1.0 to approximately 0.72 the cathodic reactant displays two phases, one being essentially pure sulfur and the other being sulfur saturated alkali metal polysulfide in which the molar ratio of sulfur to alkali metal is about 5.2:2. When the device is discharged to the point where the mole fraction of sulfur is about 0.72 the cathodic reactant becomes one phase in nature since all elemental sulfur has formed polysulfide salts. As the device is discharged further, the cathodic reactant remains one phase in nature and as the mole fraction of sulfur drops so does the open circuit voltage corresponding to the change in the potential determining reaction. Thus, the device continues to discharge from a point where polysulfide salts contain sulfur and alkali metal in a molar ratio of approximately 5.2:2 to the point where polysulfide salts contain sulfur and alkali metal in a ratio of about 3:2. At this point the device is fully discharged.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus, electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the electrode and are conducted through the electrode to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and tubular barrier to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the tubular barrier and leaves behind molten elemental sulfur.
Many of the electrical conversion devices discussed above, including the alkali metal/sulfur secondary cells or batteries, and a number of materials suitable for forming the cation-permeable barriers thereof are disclosed in the following U.S. Pat. Nos. 3,404,035; 3,404,036; 3,413,150; 3,446,677; 3,458,356; 3,468,709; 3,468,719; 3,475,220; 3,475,223; 3,475,225; 3,535,163; 3,719,531 and 3,811,493.
Among the materials disclosed in the prior art, including the above patents, as being useful as the cation-permeable barrier are glasses and polycrystalline ceramic materials. Among the glasses which may be used with such devices and which demonstrate an unusually high resistance to attack by molten alkali metal are those having the following composition: (1) between about 47 and about 58 mole percent sodium oxide, about 0 to about 15, preferably about 3 to about 12, mole percent of aluminum oxide and about 34 to about 50 mole percent of silicon dioxide; and (2) about 35 to about 65, preferably about 47 to about 58, mole percent sodium oxide, about 0 to about 30, preferably about 20 to about 30, mole percent of aluminum oxide, and about 20 to about 50, preferably about 20 to about 30, mole percent boron oxide. These glasses may be prepared by conventional glass making procedures using the listed ingredients and firing at temperatures of about 2700.degree. F.
The polycrystalline ceramic materials useful as cation-permeable barriers are bi- or multi-metal oxides. Among the polycrystalline bi- or multi-metal oxides most useful in the devices to which the improvement of this invention applies are those in the family of Beta-alumina all of which exhibit a generic crystalline structure which is readily identifiable by X-ray diffraction. Thus, Beta-type-alumina or sodium Beta-type alumina is a material which may be thought of as a series of layers of aluminum oxide held apart by columns of linear Al-O bond chains with sodium ions occupying sites between the aforementioned layers and columns. Among the numerous polycrystalline Beta-type-alumina materials useful as reaction zone separators or solid electrolytes are the following:
1. Standard Beta-type-alumina which exhibits the above-discussed crystalline structure comprising a series of layers of aluminum oxide held apart by layers of linear Al-O bond chains with sodium occupying sites between the aforementioned layers and columns. Beta-type-alumina is formed from compositions comprising at least about 80% by weight, preferably at least about 85% by weight, of aluminum oxide and between about 5 and about 15 weight percent, preferably between about 8 and about 11 weight percent, of sodium oxide. There are two well known crystalline forms of Beta-type-alumina, both of which demonstrate the generic Beta-type-alumina crystalline structure discussed hereinbefore and both of which can easily be identified by their own characteristic X-ray diffraction pattern. Beta-alumina is one crystalline form which may be represented by the formula Na.sub.2 0.11Al.sub.2 O.sub.3. The second crystalline is B"-alumina which may be represented by the formular Na.sub.2 0.6Al.sub.2 O.sub.3. It will be noted that the B" crystalline form of Beta-type-alumina contains approximately twice as much soda (sodium oxide) per unit weight of material as does the Beta-alumina. It is the B"-alumina crystalline structure which is preferred for the formation of the cation-permeable barriers for the devices to which improvement of this invention is applicable. In fact, if the less desirable beta form is present in appreciable quantities in the final ceramic, certain electrical properties of the body will be impaired.
2. Boron oxide B.sub.2 O.sub.3 modified Beta-type-alumina wherein about 0.1 to about 1 weight percent of boron oxide is added to the composition.
3. Substituted Beta-type-alumina wherein the sodium ions of the composition are replaced in part or in whole with other positive ions which are preferably metal ions.
4. Beta-type-alumina which is modified by the addition of a minor proportion by weight of metal ions having a valence not greater than 2 such that the modified Beta-type-alumina composition comprises a major proportion by weight of a metal ion in crystal lattice combination with cations which migrate in relation to the crystal lattice as a result of an electric field, the preferred embodiment for use in such electrical conversion devices being wherein the metal ion having a valence not greater than 2 is either lithium or magnesium or a combination of lithium and magnesium. These metals may be included in the composition in the form of lithium oxide of magnesium oxide or mixtures thereof in amounts ranging from 0.1 to about 5 weight percent.
As mentioned previously, the energy conversion devices to which the improvement of this invention applies include an alkali metal reservoir which contains the alkali metal anode-reactant and the level of which fluctuates during the operation of the device. This reservoir must be joined to the cation-permeable barrier in such a manner as to prevent both ionic and electronic current leakage between the alkali metal in the reservoir and the cathodic reaction zone. This insulation insures that the ionic conduction takes place in the cation-permeable barrier while the electronic conduction accompanying the chemical reaction follows the external shunt path resulting in useful work. Therefore, the sealing of an insulating alkali metal reservoir to the action-permeable barrier in such a manner as to prevent internal current leakage is critical to the satisfactory performance of the battery. This seal must also support the loads on the cation-permeable barrier or electrolyte assembly, should in no way introduce deleterious properties into the electrical conversion device system, and must withstand a variety of environments varying both in temperature and corrosive nature.
The seal which has been employed in the past for sealing the ceramic header or insulator to the cation-permeable seal has been a butt seal between the cylindrical cross-sections of the two tubular members. The glass normally employed for such a seal is a borosilicate glass formed from about 6 to about 11 weight percent of Na.sub.2 O, about 41 to about 51 weight percent of SiO.sub.2 and about 53 to about 59 weight percent of B.sub.2 O.sub.3. Such borosilicate glasses have a number of properties making them well suited for use as sealing components in electrical conversion devices. These properties include: (1) reasonably good chemical stability to liquid alkali metal, e.g., sodium, sulfur and various polysulfides at and above 300.degree. C; (2) good wetting to, but limited reactivity with, alumina ceramics; (3) a thermal expansion coefficient closely matched to both alpha and beta alumina ceramics; (4) easy formability with good fluid properties and low strain, annealing and melting temperatures; and (5) low electrical conductivity and hence small diffusion coefficients.
The outstanding properties of the above borosilicate glasses notwithstanding, the butt seal configuration which has been employed results in a stress concentration in the glass component while the glass is simultaneously exposed to corrosive electrode materials. Of course, failure of the glass seal will result in catastrophic failure of the energy conversion device. Since the butt seal configuration allows a large surface area of relatively thin glass (e.i., the thickness of the tubular walls sealed) to be exposed to corrosive materials, the time for diffusion of materials, such as sodium, through the glass is less than desirable. In fact, this type of glass seal effectively limits the maximum temperature at which the sealed composite assembly may operate as the conductive component in such energy conversion devices since increased operating temperature which is desirable for enhanced cell performance is accompanied by accelerated corrosion and heightened stress which limit seal life.